1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser device, and more specifically, to a method for manufacturing a distributed feedback semiconductor laser device.
2. Description of the Related Art
In a related-art distributed feedback semiconductor laser (hereinafter referred to as xe2x80x9cDFB laserxe2x80x9d) comprising a optical-wave guide ridge having a diffraction grating formed therein, when the diffraction grating is formed as a part of the steps in the method for manufacturing a semiconductor laser, etching of a diffraction grating layer having the same width as the ridge width is performed using an SiO2 film mask pattern having an opening of a diffraction grating shape wider than the ridge width, and a resist pattern formed on the SiO2 film mask pattern having an opening with the same width as the ridge width extending in the direction of the optical wave guide.
A related-art method for manufacturing a diffraction grating will be described.
FIGS. 33, 35, and 39 are plans of a semiconductor laser showing a step in a related-art method for manufacturing a semiconductor laser, for example, disclosed in Japanese Patent Application No. 2000-352450; FIG. 34 is a sectional view of the semiconductor laser along line 34xe2x80x9434 in FIG. 33; FIG. 36 is a sectional view of the semiconductor laser along line 36xe2x80x9436 in FIG. 35; FIG. 37 is a sectional view of the semiconductor laser along line 37xe2x80x9437 in FIG. 35; FIG. 38 is a sectional view of the semiconductor laser along line 38xe2x80x9438 in FIG. 35; and FIG. 40 is a sectional view of the semiconductor laser along line 40xe2x80x9440 in FIG. 39.
First, on an n-type InP substrate (hereinafter xe2x80x9cn-typexe2x80x9d and xe2x80x9cp-typexe2x80x9d are described as xe2x80x9cn-xe2x80x9d and xe2x80x9cp-xe2x80x9d, respectively), an n-InP clad layer, an n-AlInAs clad layer, an n-AlGaInAs light-confinement layer, an AlGaInAs quantum-well layer, a p-AlGaInAs light-confinement layer, a p-AlInAs clad layer, a p-InP layer, a p-InGaAsP layer, and a p-InP layer are laminated and grown. Then, an SiO2 insulating film is formed, and a resist film is grown thereon.
Next, EB exposure is performed at a pitch p10 of about 2000 xc3x85 and a width of 10 xcexcm (xc2xd the pitch p10, 1000 xc3x85 for the exposed portion) and development is performed to form a resist pattern, the SiO2 insulating film is etched using the resist pattern to form an SiO2 insulating film pattern, and the resist pattern is removed. The result of this process is shown in FIGS. 33 and 34.
In FIGS. 33 and 34, the reference numeral 122 denotes an SiO2 insulating film pattern, and 122a denotes an SiO2 insulating film opening. The length a10 of the SiO2 insulating film opening 122a corresponds to the exposure width of EB, 10 xcexcm, and the width w10 of the SiO2 insulating film opening 122a corresponds to the xc2xd the EB exposure pitch p10, 1000 xc3x85.
In FIG. 34, the reference numeral 101 denotes an n-InP substrate, 102 denotes an n-InP clad layer, 103 denotes an n-AlInAs clad layer, 104 denotes an n-AlGaInAs light-confinement layer, 105 denotes an AlGaInAs quantum-well layer, 106 denotes a p-AlGaInAs light-confinement layer, 107 denotes a p-AlInAs clad layer, 108 denotes a p-InP layer, 110 denotes a p-InGaAsP layer, and 121 denotes a p-InP layer.
Next, referring to FIGS. 35, 36, and 37, a resist is applied onto the SiO2 insulating film pattern 122, and a resist pattern 124 having a resist pattern opening 124a is formed using photoengraving along the row of the SiO2 insulating film openings 122a. The result of this step is shown in FIGS. 35, 36, 37, and 38.
In FIG. 35, the width w20 of a resist pattern opening 124a is 1.8 xcexcm, which is the same as the width of the optical-wave guide ridge formed later.
FIG. 37 shows a cross section of the portion where the p-InP layer 121 is covered with the SiO2 insulating film pattern 122; and FIG. 38 shows a cross section of the portion where an area of the p-InP layer 121 is not covered with the SiO2 insulating film pattern 122, and exposed on the surface at the width w20 of the resist pattern opening 124a. 
Next, areas of the p-InP layer 121 and the p-InGaAsP layer 110 not covered by the SiO2 insulating film pattern 122 and the resist pattern 124, that is the area exposed on the surface in the sectional view of FIG. 38 are etched by dry etching using the SiO2 insulating film pattern 122 and the resist pattern 124 as a mask, and using methane gas and hydrogen plasma as etching media, to expose the p-InP layer 108. Then, the SiO2 insulating film pattern 122 and the resist pattern 124 are removed. The result is shown in FIGS. 39 and 40.
Thereafter, a p-InP layer is grown and filled to form a diffraction grating layer composed of a p-InGaAs/p-InP layer.
In the related-art method for manufacturing a diffraction grating, when dry etching is performed using an SiO2 insulating film pattern 122 and a resist pattern 124 as a mask and using methane gas and hydrogen plasma as etching media, methane gas and hydrogen plasma, which are etching media, may react with the resist, which is an organic substance, to change the concentration of methane gas and hydrogen plasma, which determine the etching rate; and in some cases, the depth of etching in the direction along the width w20 of the resist pattern opening 124a may lack uniformity.
FIG. 41 is a schematic diagram showing the distribution of depth in the direction along the width w10 of the SiO2 insulating film openings 122a of a related-art diffraction grating; and FIG. 42 is a schematic diagram showing the distribution of depth in the direction along the width w20 of the resist pattern opening 124a. 
As seen from FIG. 42, the depth of the grooves closer to the resist becomes shallow due to the lowered etching rate. Also, since the reaction of the resist with methane and hydrogen plasma changes depending on the surface conditions of the resist, dependence of the etching rate on the surface conditions of the resist may occur, and the etching rate may differ between lots. For this reason, there was difficulty in forming a diffraction grating having an even thickness in the width direction of the ridge wave guide, and fluctuation in the laser characteristics of semiconductor lasers, resulting in lowering of the yield of semiconductor lasers.
The known techniques include Japanese Patent Application Laid-Open No. Hei.6-291408 (1994), which discloses the use of a resist, oxide film, or nitride film as a material of a pattern forming film for forming diffraction gratings.
Japanese Patent Application Laid-Open No. Sho.62-165392 (1987) discloses a method for separately etching regions with inverted periodicity, when a xcex/4 shifted diffraction grating is formed, using a patterning layer of an SiO2 oxide film and a patterning layer of an aluminum film.
Furthermore, Japanese Patent Application Laid-Open No. Sho.62-139503 (1987) discloses a method for forming a diffraction grating in a specific region by forming the mask pattern of a first photoresist having a window corresponding to the specific region laminated with the mask pattern of a second photoresist that does not react with the first photoresist, and by using these two types of mask patterns as masks.
The present invention has been made to overcome the above-described drawbacks and disadvantages of the related art. It is an object of the present invention to provide a method for manufacturing a semiconductor laser that can easily manufacture a semiconductor laser having diffraction gratings of an even thickness, and having uniform laser characteristics.
According to one aspect of the invention, there is provided a method for manufacturing a semiconductor laser device comprising: a first step of sequentially laminating on a semiconductor substrate of a first conductivity type, a first clad layer of a first conductivity type, an active layer, a first second clad layer of a second conductivity type, a semiconductor layer of the second conductivity type with an index of refraction different from the index of refraction of the second clad layer, and a second second clad layer of the second conductivity type; a second step of forming a first insulating film of a Si-based substance on the surface of the second second clad layer, and forming with the first insulating film a first insulating film pattern with a plurality of first openings which have a strip shape of a predetermined length in the direction intersecting the direction of an optical wave guide, and which are arranged at intervals of a predetermined distance in the direction of an optical wave guide; a third step of forming a second insulating film of a Si-based substance over the semiconductor substrate through the first insulating film pattern, and forming with the second insulating film through the first insulating film pattern a second insulating film pattern with a second opening which has a strip shape extending in the direction of the optical wave guide, and which has a width narrower than the length of the first opening in the direction intersecting the optical wave guide direction; a fourth step of etching the second second clad layer and the semiconductor layer using the second insulating film pattern and the first insulating film pattern as masks to form a third opening passing through the semiconductor layer; and a fifth step of removing the second insulating film pattern and the first insulating film pattern, and filling the second second clad layer and the semiconductor layer through the third opening with a third second clad layer of the second conductivity type. Accordingly, the present invention is advantageous that in manufacturing a semiconductor laser device the reaction of the material for the mask pattern with the etching media is made difficult to occur in etching, and that the unstable variation of the etching rate due to the reaction of the material for the mask pattern with the etching media can be inhibited.
Therefore, since the thickness of the diffraction grating layer can be uniform, a diffraction grating having stable optical characteristics can be formed, and a DFB laser of stable laser characteristics can be formed. Thus, a DFB laser having favorable laser characteristics can be provided at a low price.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.